Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century

Significance A wide array of scientific disciplines and industries use radiocarbon analyses; for example, it is used in dating of archaeological specimens and in forensic identification of human and wildlife tissues, including traded ivory. Over the next century, fossil fuel emissions will produce a large amount of CO2 with no 14C because fossil fuels have lost all 14C over millions of years of radioactive decay. Atmospheric CO2, and therefore newly produced organic material, will appear as though it has “aged,” or lost 14C by decay. By 2050, fresh organic material could have the same 14C/C ratio as samples from 1050, and thus be indistinguishable by radiocarbon dating. Some current applications for 14C may cease to be viable, and other applications will be strongly affected. Radiocarbon analyses are commonly used in a broad range of fields, including earth science, archaeology, forgery detection, isotope forensics, and physiology. Many applications are sensitive to the radiocarbon (14C) content of atmospheric CO2, which has varied since 1890 as a result of nuclear weapons testing, fossil fuel emissions, and CO2 cycling between atmospheric, oceanic, and terrestrial carbon reservoirs. Over this century, the ratio 14C/C in atmospheric CO2 (Δ14CO2) will be determined by the amount of fossil fuel combustion, which decreases Δ14CO2 because fossil fuels have lost all 14C from radioactive decay. Simulations of Δ14CO2 using the emission scenarios from the Intergovernmental Panel on Climate Change Fifth Assessment Report, the Representative Concentration Pathways, indicate that ambitious emission reductions could sustain Δ14CO2 near the preindustrial level of 0‰ through 2100, whereas “business-as-usual” emissions will reduce Δ14CO2 to −250‰, equivalent to the depletion expected from over 2,000 y of radioactive decay. Given current emissions trends, fossil fuel emission-driven artificial “aging” of the atmosphere is likely to occur much faster and with a larger magnitude than previously expected. This finding has strong and as yet unrecognized implications for many applications of radiocarbon in various fields, and it implies that radiocarbon dating may no longer provide definitive ages for samples up to 2,000 y old.

Radiocarbon analyses are commonly used in a broad range of fields, including earth science, archaeology, forgery detection, isotope forensics, and physiology. Many applications are sensitive to the radiocarbon ( 14 C) content of atmospheric CO 2 , which has varied since 1890 as a result of nuclear weapons testing, fossil fuel emissions, and CO 2 cycling between atmospheric, oceanic, and terrestrial carbon reservoirs. Over this century, the ratio 14 C/C in atmospheric CO 2 (Δ 14 CO 2 ) will be determined by the amount of fossil fuel combustion, which decreases Δ 14 CO 2 because fossil fuels have lost all 14 C from radioactive decay. Simulations of Δ 14 CO 2 using the emission scenarios from the Intergovernmental Panel on Climate Change Fifth Assessment Report, the Representative Concentration Pathways, indicate that ambitious emission reductions could sustain Δ 14 CO 2 near the preindustrial level of 0‰ through 2100, whereas "business-as-usual" emissions will reduce Δ 14 CO 2 to −250‰, equivalent to the depletion expected from over 2,000 y of radioactive decay. Given current emissions trends, fossil fuel emission-driven artificial "aging" of the atmosphere is likely to occur much faster and with a larger magnitude than previously expected. This finding has strong and as yet unrecognized implications for many applications of radiocarbon in various fields, and it implies that radiocarbon dating may no longer provide definitive ages for samples up to 2,000 y old.
fossil fuel emissions | radiocarbon | atmospheric CO 2 | 14 C dating | isotope forensics R adiocarbon is produced naturally in the atmosphere and decays with a half-life of 5,700 ± 30 y (1-3). Fossil fuels, which are millions of years old, are therefore devoid of 14 C, and their combustion adds only the stable isotopes 12 C and 13 C to the atmosphere as CO 2 . First observed by Hans Suess in 1955 using tree ring records of atmospheric composition (4), the dilution of 14 CO 2 by fossil carbon provided one of the first indications that human activities were strongly affecting the global carbon cycle. The apparent "aging" of the atmosphere-i.e., the decreasing trend in the ratio 14 C/C of CO 2 (reported as Δ 14 CO 2 ) (5)-was interrupted in the 1950s when nuclear weapons testing produced an immense amount of "bomb" 14 C that approximately doubled the 14 C content of the atmosphere. Direct atmospheric observations began in the 1950s, capturing the rapid rise of Δ 14 CO 2 and its subsequent quasi-exponential decay as the bomb 14 C mixed into oceanic and biospheric reservoirs (6-9) (Fig. 1). Now that several decades have passed since the Partial Nuclear Test Ban Treaty and the peak in atmospheric Δ 14 CO 2 , fossil fuel emissions are once again the main influence on the long-term trend in Δ 14 CO 2 (7,10). The growth or decline of fossil fuel emissions over the coming century determines to what extent Δ 14 CO 2 will be diluted further by fossil carbon. Also important is how atmospheric Δ 14 CO 2 dilution is moderated by natural exchanges of CO 2 with the ocean and the terrestrial biosphere. Future dynamics of carbon and 14 C are simulated here using the Representative Concentration Pathways (RCPs) developed for the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (11,12), and a simple carbon cycle model with parameters constrained by 20th-century atmospheric and oceanic Δ 14 C and CO 2 observations (10, 13) (SI Text).
The simple carbon cycle model includes a one-dimensional box diffusion model of the ocean and represents the atmosphere and the biosphere as one-box carbon reservoirs (10,13,14). Exchanges of carbon and 14 C are governed by a small number of model parameters (Table S1). Multiple simulations were run using various parameter sets selected by their representation of Δ 14 C and inventories of CO 2 and bomb 14 C (15-17). Atmospheric CO 2 concentration and fossil fuel and land use fluxes were prescribed by the RCPs, which include historical data through 2005. To match the prescribed atmospheric CO 2 concentration, the residual of carbon emissions and atmospheric and oceanic accumulation was added to the biospheric reservoir (single deconvolution). Atmospheric Δ 14 CO 2 was prescribed by observations until 2005, then predicted by model fluxes from 2005 to 2100 (SI Text).

Results
From its present value of ∼20‰ (18), which signifies a 2% enrichment in 14 C/C of CO 2 above preindustrial levels, Δ 14 CO 2 is certain to cross below the preindustrial level of 0‰ by 2030, but potentially as soon as 2019 ( Fig. 1, Table S2). After 2030, simulated Δ 14 CO 2 trends diverge according to the continued growth, slowing, or reversal of fossil fuel CO 2 emissions in the RCP scenarios. Distinct patterns are simulated for different RCPs despite the range of model parameters used, indicating the Significance A wide array of scientific disciplines and industries use radiocarbon analyses; for example, it is used in dating of archaeological specimens and in forensic identification of human and wildlife tissues, including traded ivory. Over the next century, fossil fuel emissions will produce a large amount of CO 2 with no 14 C because fossil fuels have lost all 14 C over millions of years of radioactive decay. Atmospheric CO 2 , and therefore newly produced organic material, will appear as though it has "aged," or lost 14 C by decay. By 2050, fresh organic material could have the same 14 C/C ratio as samples from 1050, and thus be indistinguishable by radiocarbon dating. Some current applications for 14 C may cease to be viable, and other applications will be strongly affected. fossil fuel emissions scenario is the determining factor for longterm Δ 14 CO 2 trends in these simulations rather than the rates of carbon cycling.
The business-as-usual emissions in RCP8.5 reduce Δ 14 CO 2 more rapidly and more dramatically than the other RCPs: Δ 14 CO 2 is less than −100‰ by 2050 and reaches −250‰ in 2100, which means that atmospheric CO 2 in 2100 is as depleted in 14 C as the "oldest" part of the ocean (19) (Fig. 2).
The simulated trends in atmospheric Δ 14 CO 2 propagate to other carbon reservoirs through natural carbon exchanges (Fig.  2). Reduction of fossil fuel emissions in RCP2.6 leads to nearly steady Δ 14 C in atmospheric, biospheric, and upper ocean carbon in 2100, slightly elevated above preindustrial levels.
As atmospheric Δ 14 CO 2 decreases strongly in the higheremission scenarios, it becomes much more depleted in 14 C than actively overturning carbon in the oceanic and biospheric reservoirs (shown for RCP8.5 in Fig. 2). After 2050, the simulated airsea gradient of Δ 14 C in RCP8.5 is −50‰, opposite of the preindustrial gradient, and by 2100 atmospheric Δ 14 CO 2 is 150‰ lower than Δ 14 C at midthermocline depths of 600 m. Reversal of Δ 14 C gradients causes natural exchanges to transfer 14 C from the ocean to the atmosphere (Fig. S1), as shown by Caldeira et al. (20). Caldeira et al. (20) used a similar carbon cycle model with the IS92-A scenario from the IPCC Third Assessment Report (21), which is comparable to RCP6.0. Emissions over the past 10 y have outpaced the IS92-A and RCP6.0 scenarios, and are currently on track to follow RCP8.5 (22). Δ 14 CO 2 is nearly 50‰ lower in 2050 and 100‰ lower in 2100 in RCP8.5 compared with RCP6.0 ( Fig. 1), suggesting that fossil fuel emissions are likely to reduce Δ 14 CO 2 much faster than expected from the IS92-A scenario used by Caldeira et al. (20). As a result, the atmospheric inventory of 14 C could approach the peak 14 C inventory from the early 1960s later this century (Fig. S1).  (Table S2). The right axis shows the conventional radiocarbon age of a carbon-containing specimen with the same radiocarbon content, calculated by 8033 * ln (Δ 14 C/1,000 + 1). Filled areas indicate the range simulated for different sets of model parameters, each consistent with 20th-century atmospheric and oceanic Δ 14 C and CO 2 observations, within their uncertainties (10) (SI Text).

Fig. 2.
Simulation of radiocarbon in various carbon reservoirs for the low-emission and business-as-usual RCPs. Simulated Δ 14 C in biospheric and four ocean carbon reservoirs, surface mixed layer, 300, 600, and 3,500 m in RCP2.6 (Left) and RCP8.5 (Right). The black line shows the midrange value of simulated atmospheric Δ 14 CO 2 (full simulated range of Δ 14 CO 2 is shown in Fig. 1).

Discussion
Though these simulations indicate tremendous changes to the global radiocarbon cycle and the use of radiocarbon in earth science and biogeochemical research, the potential impacts of future trends in Δ 14 CO 2 are much broader. Radiocarbon is currently used in a wide array of scientific and industrial applications, many of which exploit its radioactive decay to determine the age of carbon-containing specimens through radiocarbon dating. Atmospheric CO 2 is presently aging at a rate of ∼30 y·y −1 , and by 2050 the atmosphere could appear to be 1,000 y old in conventional radiocarbon age (Fig. 1). In 2100, atmospheric CO 2 in RCP8.5 has the same radiocarbon content as a specimen that has undergone 2,000 y of radioactive decay-i.e., a specimen originating around AD 100. The aging of the atmosphere predicted by these simulations has the potential to severely impact the use of radiocarbon dating. Within the next 85 y, the atmosphere may experience Δ 14 CO 2 corresponding to conventional ages from within the historical period encompassing the Roman, Medieval and Imperial Eras. For archaeological or other items that are found without sufficient context to rule out a modern origin, radiocarbon dating will give ambiguous results.
Radiocarbon has various applications in isotope forensics (23). Some applications use the presence of elevated Δ 14 C in a sample to distinguish its origin to be subsequent to 1950, whereas other applications use the rapid changes in Δ 14 CO 2 after 1963 (the socalled "bomb curve") to distinguish the year of origin of a sample more precisely. These techniques have been used to test vintages of wine and whisky, identify the age of human remains, and detect illegal ivory trading (23)(24)(25). As with radiocarbon dating, forthcoming Δ 14 CO 2 changes are likely to introduce ambiguity into these techniques, and the presence of elevated Δ 14 CO 2 will not identify samples with recent origins beyond ∼2030.
If the lower-emission RCPs 2.6 or 4.5 were followed instead of business-as-usual RCP8.5, the simulated stabilization of Δ 14 CO 2 ( Fig. 1) would hinder the use of radiocarbon in fields such as ecology and physiology. These applications take advantage of the decreasing trend in Δ 14 CO 2 since the 1960s to evaluate the decadal-scale rate of turnover of carbon in soil compounds (26) or human cells (27), for example. Annual changes in Δ 14 CO 2 could become undetectable in two to three decades if CO 2 emissions are rapidly reduced, thereby limiting the use of these applications.
Another prominent application for radiocarbon involves the identification of CO 2 emitted by fossil fuel combustion in urban or continental regions (28,29). Here too, the projected atmospheric changes will have a major impact. The sensitivity of Δ 14 CO 2 to local additions of fossil fuel-derived CO 2 depends on the concentration of atmospheric CO 2 and the Δ 14 C disequilibrium between atmospheric CO 2 and fossil carbon (being 14 C-free, the Δ 14 C of fossil fuels is −1,000‰). Neglecting other effects, the sensitivity of Δ 14 CO 2 to fossil fuel-derived CO 2 is approximated by α ∼ (−1,000‰ − Δ 14 CO 2 )/CO 2 . The ratio α is presently −2.6‰ ppm −1 , but it drops to −1.6‰ ppm −1 by 2050 and to −0.8‰ ppm −1 by 2100 in RCP8.5; this suggests that measurement precision will have to increase by approximately a factor of 2 in the next few decades simply to maintain current detection capabilities for fossil fuel-derived CO 2 .
Simulated trends in Δ 14 CO 2 therefore motivate new efforts to improve precision in radiocarbon measurements and to develop ancillary measurements that can help resolve ambiguity in radiocarbon analyses. The development of accelerator mass spectrometry (AMS) in the 1980s dramatically reduced the amount of time and the amount of carbon needed for analysis, compared with traditional decay counting techniques (30,31). Further improvement in the precision of 14 C detection by AMS is needed, as well as reductions in other sources of uncertainty (32), which may change as atmospheric Δ 14 CO 2 decreases. Ever-higher requirements in precision will also need to be met by alternative measurement techniques currently in development (33).
Fossil fuel emissions are now increasing faster than ever before. Continued business-as-usual growth in emissions will cause the atmosphere to approach a 1,000-y-old radiocarbon age by 2050 and a 2,000-y-old age by 2100. The application of radiocarbon in a wide array of disciplines implies that changing atmospheric radiocarbon content will have farreaching impacts.
ACKNOWLEDGMENTS. The author thanks S. Frolking, P. Reimer, E. Kato, and T. Guilderson for helpful discussions, and the RCP Task Group for providing CO 2 concentration and emission scenarios. Support for this work was provided by the European Commission through a Marie Curie Career Integration Grant.